hello,
currently in salome you can not add layers to a structure like this: check this post I made some time ago.Bug SMESH hexahedron + layer generation issue with internal faces of blocks
so yeah, for the blocking of the geoemtry you have (i know it as i made it myself) the boundary layer will fail in any case (the 32 % is quite commun, it is calculating, at one point it will finish, but i know for certenty that you will have collapsed layers in any case.)
only solution is to change the blocking of the geometry. search a little bit for pillowing keyword. if you are using openfoam for your simulation, you can use your mesh+snappy only with viscous layer true to add them.
regards